The invention relates to a process for the purification of crude diaminomaleonitrile which is produced by using hydrogen cyanide as a raw material.
Diaminomaleonitrile is a useful compound as an intermediate for glycine, 2-amino-4,5-dicyanoimidazole, 4-aminoimidazole-5-carboxamide, xanthine, hypoxanthine, cafeine, theophilline, adenine, guanine and the like.
The development in the method for the purification of diaminomaleonitrile is very important because a highly pure diaminomaleonitrile is required when it is used as a intermediate for food additives or medicines.
Methods by means of sublimation, formation of complex, and extraction are well-known for the purification of diaminomaleonitrile. The sublimation method is disadvantageous due to a low yield. Also, the method by formation of complex is disadvantageous due to the complicated operation and is difficult to be employed for an industrial process. The extraction method is usually employed for the purification of diaminomaleonitrile.
Japanese Patent Publication (before examination) No. 30319/1974 discloses a process for the purification of diaminomaleonitrile by extraction, wherein diaminomaleonitrile is extracted with an ester of acetic acid the amount of which is 16 to 20 times the weight of a crude diaminomaleonitrile, and after adding the same amount of hydrocarbon as the ester of acetic acid to the extract, the ester is removed by distillation to precipitate and separate diaminomaleonitrile having a purity of 95 to 98%.
According to Japanese Patent Publication (before examination) No. 30321/1974, a mixed solvent consisting of a group of ketones, alcohols, nitriles and ethers, and a group of hydrocarbons and halogenated hydrocarbons is used as an extraction solvent. The amount of the solvent is 33 to 40 times the weight of a crude diaminomaleonitrile and diaminomaleonitrile having a purity of 93 to 95% is obtained.
Japanese Patent Publication (before examination) No. 81818/1973 also discloses an extraction method. In the method, a crude diaminomaleonitrile having a purity of about 90% is washed with a hydrocarbon or a halogenated aliphatic hydrocarbon by an amount of 15 times the weight of the crude diaminomaleonitrile and then, a purified diaminomaleonitrile having a purity of 99 to 99.5% is obtained by extraction with methyl acetate, ethyl acetate, isopropyl alcohol or the like the amount of which is 15 to 25 times the weight of the crude diaminomaleonitrile.
When diaminomaleonitrile is produced by the tetramerization of hydrogen cyanide, it is unavoidable that a higher polymer than tetramer is formed simultaneously with the production of diaminomaleonitrile. In addition, said higher polymer is liable to be contained in a purified product by extraction due to its solubility property which is similar to that of diaminomaleonitrile.
Consequently, the selectivity to diaminomaleonitrile must be given greater consideration than the solubility of diaminomaleonitrile when selecting an extraction solvent for the purification of diaminomaleonitrile, and the amount used of the extraction solvent is very large in the conventional extraction method through the purified product has a considerbly high purity.